implement ipv4v6 context type

The PDP Context type ipv4v6 isn't yet fully supported.When trying to request such context, the GGSN is rejecting it with GTPCAUSE_UNKNOWN_PDP (220).

The PDP context creation fails within create_context_ind() in ggsn/ggsn.c:555,because in46a_from_eua() can not decode the requested PDP EUA Type 0xf1 0x8d.The in46a lib need also to be extended to support dual stack.

Initial support implemented in 2d6a69e69a4b4cb2b8cc63c4810dae44e5a4d8f6 which already permits creating ipv4v6 contexts in apn which have ipv4 and ipv6 pools setup (such as inet46 in osmo-ggsn.cfg example)